DESCRIPTION (applicant's abstract): The proposed project will study the influence of steroid hormones upon reproductive behavior and the morphology of neurons which mediate that behavior, by focusing upon a simple neuromuscular system in adult male rats. Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) innervate the striated perineal muscle bulbocavernosus (BC). The BC is active during copulation and aids the formation and removal of cervical plugs which are crucial for reproductive success in this species. The BC is also active during penile flips that can be mediated by the isolated spinal cord. Androgen manipulations in adulthood alter both the reflexes and several neuroanatomical measures of the SNB, including soma size, nuclear size, dendritic extent and the size of the target muscles. Experiments will be conducted to determine which cellular population(s) interact with androgen in order to alter neural morphology and therefore behavior. These experiments include the use of mosaic analysis the study of how neurons with different genetic backgrounds, within the same individual animal, respond differently to steroid hormone. We have developed a method of identifying, and studying, within a single animal, SNB motoneurons which are androgen-sensitive and their neighboring SNB motoneurons which are androgen-insensitive. Only after the primary sites of androgen action are ascertained can further experiments be performed to understand the cellular mechanisms behind these changes in neural structure and the consequences for their function. These studies should provide further understanding of reproductive behaviors and the means by which hormones can alter behavior. Thus they could improve understanding of the reproductive process and suggest therapies for reproductive failure. They may also shed light on the mechanisms by which steroid hormones alter human development and behavior. Furthermore, by studying the mechanisms of neural plasticity, a greater understanding of learning and memory processes may be gained.